static S ai_parse_simpleArrow(S s, Pair<S> sep) {
  L<S> tok = javaTok(s);
  int i = tok_indexOfSimpleArrow(tok);
  if (i < 0) null;
  ret enclose(sep, joinSubList(tok, 0, i-1))
    + joinSubList(tok, i-1, i+2)
    + enclose(sep, joinSubList(tok, i+2));
}